Hi,
Having gone through Virtues and Flaws for Spontaneous Casting, both exhaustively and exhaustingly, and have having succumbed to someone's casting of Asperger's Search for the Missing Detail, I'm gonna talk about these.
The benchmark against which I measure these Virtues is Good Parens. This Virtue provides 60xp plus 30 spell levels. This can be considered the "principal," the xp value of one virtue point. A virtue that grants xp is like capital sunk into an enterprise. How long does it take the virtue to pay for itself?
Few of these virtues are worth anything in a saga with little advancement.
(For the sake of sanity, I am assuming the following rules about the 1xp gained per season of useful Correspondence: 1) The 1 xp gained is not considered gained from either a book or a teacher. 2) It does not trigger Good Reader, for an additional 3xp. 3) It does not trigger Study Bonus, for an additional 2xp. 4) It is not augmented by anyone's Good Teacher. 4) The xp must be taken in an Art or Ability for which xp is already being gained this season through the season's primary activity, which represents its being "on a theme that is associated with the magus's research or writing." Thus, this 1xp cannot trigger Secondary Insight or Elemental Magic. 5) An Affinity might apply to this 1xp, but does so together with other xp gained rather than being calculated a second time on this xp.)
Book Learner: I see this as the king of xp-granting virtues. For a minor virtue, pretty much any season spent reading--and that will be most seasons in most sagas--comes with 3 extra xp. It works for Arts and Abilities, anything for which there is a text! In most sagas, reading books is already the most efficient way to gain xp, which reinforces the tendency to read your way to Ultimate Power. Around 9xp/year for a minor virtue? Perhaps 1000xp over a career? For doing exactly what you were going to do anyway? Sweet. This virtue pays back its xp in 20 seasons of use. Let's call it 7 years. Nice!
Affinity: Each only works on a single Art or Ability, and grants xps in proportion to those allocated to the relevant Art or Ability. So an Affinity is limited. It is further limited in that xp put in one place offers diminishing returns. On the positive side, it applies to all sources of xp, and specialization is good! Over the course of a career, a specialist can easily end up throwing more than 450 points into an Art (to reach 30) making this virtue worth 150xp, and probably not more than 200. If a magus lives 100 years, that's a pretty poor return. What makes Affinities great is that they also work during character creation. A character who dumps 80 points into an Art immediately receives 40xp, the benefits of a starting Art at 15, and can repay the principal quickly, depending on what he does. Or, dump 140 points into the Art and start at 20, having already done as well as Strong Parens (10 extra xp versus 30 spell levels) and with only profit remaining.
Free Study: Like Book Learner, but for vis. In most sagas, a magus will never get to study from vis. In those where magi do study from vis, it will probably take many decades to use this virtue 20 times. Usually worth avoiding.
Study Bonus: This works with books and vis, but only provides a +2 bonus, only works with Arts and requires that study be done in places that become increasingly inconvenient, dangerous and potentially disruptive in proportion to the Art score being improved. Potentially worth taking after Book Learner, but it pays for itself more slowly, even if the covenant is willing to let you study those precious tractatuseses inside the volcano.
Secondary Insight: This is a major virtue. It only works when studying Arts from books, teachers and vis, in which case it yields 2xp or 4xp. It is better than Book Learner in that it also works with teachers and vis, but worse in that it only works with Arts. Its xp yield is comparable, despite being a major virtue. Like Book Learner, it does not affect xp gained during character creation, Practice, Adventure or Exposure. Overall it's as good as Book Learner, for thrice the cost. Figure 21 years or more to break even on this one. Worth avoiding, unless your saga has house rules or creative readings of existing rules that allow SI to trigger more than once per season.
Elemental Magic: This one bears thinking about. It only works with the four elements, so it's bad for characters who aren't very interested in these. This virtue grants 3xp in any season spent studying an element, which makes it strictly worse than Book Learner or Secondary Insights when reading, and worse than SI when SI works. It also does nothing during character creation. But EM can trigger twice from Exposure, and four times from Adventure. So.... how often will your character want to use Exposure or Adventure? A mere 5 seasons of Adventure or ten seasons of Exposure multiplied by 3 (because this is a Major Virtue) can pay this one off. Most characters prefer to read, but Exposure and Adventure no longer completely suck. Lots of ifs here.
Flawless Magic: This virtue provides 5xp for every Formulaic and Ritual spell learned. It also doubles the xp gained for practicing spells for Mastery. A tricky virtue, this one. It can provide many xp, but are they all worthwhile? Some spells are not worth mastering; should these xp count? A magus who learns 50 formulaic spells during his career gains 250xp from that. If he raises five of these masteries to level 5, he gains ((75-5)/2)*5 = 165xp from that. If he raises ten more of these to level 2, he gains 50xp from that. We're looking at up to 500xp gained from this virtue over the course of a career. This compares unfavorably with Book Learner, especially since some of these xps are in spells where Mastery isn't very important, and since Book Learner applies to books about Mastering spells. Yet the xp value is deceptive. Mastery texts are easy to find for some spells but not others. A magus who wants to master his Secret Ritual or rare spell is stuck with Practice, Adventure and Exposure, all normally very slow. The value of FM in this case might better be measured in seasons: Practicing to reach Mastery 1/2/3/4/5/6 saves a magus with FM 1/2/3/5/8/11 seasons, that could be spent, say, reading texts (using Book Learner ). How many spells will your magus master?
Learn Ability From Mistakes: At most, this will trigger once per session per instance of this virtue, which means it will pay for itself in no less than 12 sessions. Depending on your saga, this rate of xp gain is either incredibly fast or incredibly slow, with a maximized value in sagas that run often and last long in real time, with time passing slowly in game time. Its value also depends on whether your SG is going to give you credit for failing by exactly one point as often as it happens.
Independent Study: This one is from the Merinita section of HoH:MC, and provide an extra 3xp for Adventure and 2xp for Practice. If your character does this often, great. In most sagas, however, reading books is more efficient even without Book Learner. In the same way that 3xp extra when studying from vis is less valuable than 3xp extra when studying from text, because one source is better than the other, so too with this Virtue. It is not efficient for most characters, but might be essential for some concepts. Naturally, in sagas with house rules and special arrangements, things might be different.
Puissant Art/Ability: This doesn't grant xps directly, but its current xp value can always be determined precisely. For an Art score of X, it is worth 6+3X xp. For an Ability, it is worth 5(3+2*X) xp. It breaks even with Good Parens when a natural Art score is 18 or when a natural Ability score is 5. It will almost never provide more than 100xp of value. This isn't very xp efficient, and never worth taking except for specialists, for whom it lets them be who they are early, a subtle benefit not to be underestimated.
Anyway,
Ken